Down-regulation of an ankyrin repeat-containing protein, V-1, during skeletal muscle differentiation and its re-expression in the regenerative process of muscular dystrophy

Preparation of decellularized biphasic hierarchical myotendinous junction extracellular matrix for muscle regeneration

Muscle injury and defect affect people's quality of life, and effective treatment is lacking. Herein, we generated a scaffold to obtain decellularized porcine Achilles tendon myotendinous junction (D-MTJ) extracellular matrix (ECM) with well-preserved native biphasic hierarchical structure, biological composition, and excellent mechanical properties for muscle regeneration. The combined use of potassium chloride, potassium iodide, Triton-X 100, and sodium-dodecyl sulfate (SDS) can completely remove the main immunogenicity, while maintaining the major biological components and microstructure. The specific biomechanics of D-MTJ is comparable to the native muscle-tendon physiological conditions. Additionally, the D-MTJ ECM scaffold induced minimal immunological reaction (histology analysis) through rat subcutaneous implantation. Moreover, in vitro, muscle satellite cells adhered, proliferated, and infiltrated into the D-MTJ scaffold, and myofiber-like cell differentiation was observed as shown by increased expression of myogenesis-related genes during culture. In vivo, newly formed myofibers were observed in a muscle defect model with D-MTJ orthotopic transplantation, while the control group presented mostly with fibrous tissue deposition. Additionally, the number of Myod and MyHC-positive cells in the ECM scaffold group was higher at day 30. We preliminary explored the mechanisms underlying D-MTJ-mediated muscle regeneration, which may be attributed to its specific biphasic hierarchical structure, bio-components, and attractiveness for myogenesis cells. In conclusion, our findings suggest the D-MTJ ECM scaffold prepared in this study is a promising choice for muscle regeneration.

STATEMENT OF SIGNIFICANCE

This study is the first to use decellularization technology obtaining the specifically decellularized myotendinous junction (D-MTJ) with well-preserved biphasic hierarchical structure and constituents, excellent mechanical properties and good biocompatibility. The D-MTJ was further proved to be efficient for muscle regeneration in vitro and in vivo, and the underlying mechanisms may be attributed to its specifically structure and constituents, improved myogenesis and good preservation of repair-related factors. Our study may provide basis for the decellularization of other biphasic hierarchical tissues and a platform for further studies on muscle fiber and tendon integrations in vitro.

Disuse Atrophy Accompanied by Intramuscular Ectopic Adipogenesis in Vastus Medialis Muscle of Advanced Osteoarthritis Patients

Muscle dysfunction is the most important modifiable mediating factor in primary osteoarthritis (OA) because properly contracting muscles are a key absorber of forces acting on a joint. However, the pathological features of disuse muscle atrophy in OA patients have been rarely studied. Vastus medialis muscles of 14 female patients with OA (age range, 69 to 86 years), largely immobile for 1 or more years, were obtained during arthroplastic surgery and analyzed histologically. These were compared with female patients without arthritis, two with patellar fracture and two with patellar subluxation. Areas occupied by myofibers and adipose tissue were quantified. Large numbers of myofibers were lost in the vastus medialis of OA patients. The loss of myofibers was a possible cause of the reduction in muscle strength of the operated on knee. These changes were significantly correlated with an increase in intramuscular ectopic adipose tissue, and not observed in knees of nonarthritic patients. Resident platelet-derived growth factor receptor α-positive mesenchymal progenitor cells contributed to ectopic adipogenesis in vastus medialis muscles of OA patients. The present study suggests that significant loss of myofibers and ectopic adipogenesis in vastus medialis muscles are common pathological features of advanced knee OA patients with long-term loss of mobility. These changes may be related to the loss of joint function in patients with knee OA.

Impairment of cold injury-induced muscle regeneration in mice receiving a combination of bone fracture and alendronate treatment

Alendronate, a nitrogen-containing bisphosphonate, is well established as a treatment for osteoporosis through regulation of osteoclast activity. Previously, the pharmacological effects of bisphosphonates on cells outside the bone environment have been considered irrelevant because bisphosphonates target bone. Here we show that administration of alendronate impairs muscle regeneration in mice after bone fracture. A series of injections of alendronate alone or bone fracture alone did not affect muscle regeneration induced by cold injury. In contrast, alendronate treatment plus bone fracture severely impaired the regeneration of muscle that closely contacts the bone fracture site after cold injury. After cold injury, M-cadherin-positive myogenic cells disappeared in the damaged muscle areas of mice receiving the combination of alendronate treatment and bone fracture. The present results suggest that the muscle regeneration capacity is impaired by bone fracture in mice receiving alendronate treatment. The present research on the pharmacological effects of alendronate on muscle regeneration will aid in understanding of the in vivo action of alendronate on skeletal muscles.

Sparing of the dystrophin-deficient cranial sartorius muscle is associated with classical and novel hypertrophy pathways in GRMD dogs

Both Duchenne and golden retriever muscular dystrophy (GRMD) are caused by dystrophin deficiency. The Duchenne muscular dystrophy sartorius muscle and orthologous GRMD cranial sartorius (CS) are relatively spared/hypertrophied. We completed hierarchical clustering studies to define molecular mechanisms contributing to this differential involvement and their role in the GRMD phenotype. GRMD dogs with larger CS muscles had more severe deficits, suggesting that selective hypertrophy could be detrimental. Serial biopsies from the hypertrophied CS and other atrophied muscles were studied in a subset of these dogs. Myostatin showed an age-dependent decrease and an inverse correlation with the degree of GRMD CS hypertrophy. Regulators of myostatin at the protein (AKT1) and miRNA (miR-539 and miR-208b targeting myostatin mRNA) levels were altered in GRMD CS, consistent with down-regulation of myostatin signaling, CS hypertrophy, and functional rescue of this muscle. mRNA and proteomic profiling was used to identify additional candidate genes associated with CS hypertrophy. The top-ranked network included α-dystroglycan and like-acetylglucosaminyltransferase. Proteomics demonstrated increases in myotrophin and spectrin that could promote hypertrophy and cytoskeletal stability, respectively. Our results suggest that multiple pathways, including decreased myostatin and up-regulated miRNAs, α-dystroglycan/like-acetylglucosaminyltransferase, spectrin, and myotrophin, contribute to hypertrophy and functional sparing of the CS. These data also underscore the muscle-specific responses to dystrophin deficiency and the potential deleterious effects of differential muscle involvement.

Molecular characterization, expression patterns and subcellular localization of Myotrophin (MTPN) gene in porcine skeletal muscle

Myotrophin (MTPN) is an effective growth factor in promoting skeletal muscle growth in vitro and vivo and has been purified from porcine skeletal muscle. However, in pigs, the information on MTPN gene is very limited. In this study, we cloned cDNA sequences and analyzed the genomic structure of porcine MTPN gene. The deduced amino acid sequence of porcine MTPN contains two the ankyrin repeat domains. RT-PCR analysis revealed that porcine MTPN gene was widely expressed in many tissues, a high expression level was observed in the spleen, liver and uterus, and transient transfection indicated that porcine MTPN proteins was located in cytoplasms within Pig Kidney Epithelial cells (PK15). Quantitative real-time PCR (qRT-PCR) analyses showed that MTPN expression peaked at embryonic 65 day post conception (dpc). During postnatal muscle development, MTPN expression was down-regulated from the 3 day to the 180 day in Yorkshire pigs. This result suggests that the MTPN gene may be important gene for skeletal muscle growth and provides useful information for further studies on its roles in porcine skeletal muscle.

CDK4 and cyclin D1 allow human myogenic cells to recapture growth property without compromising differentiation potential

In vitro culture systems of human myogenic cells contribute greatly to elucidation of the molecular mechanisms underlying terminal myogenic differentiation and symptoms of neuromuscular diseases. However, human myogenic cells have limited ability to proliferate in culture. We have established an improved immortalization protocol for human myogenic cells derived from healthy and diseased muscles; constitutive expression of mutated cyclin-dependent kinase 4, cyclin D1 and telomerase immortalized human myogenic cells. Normal diploid chromosomes were preserved after immortalization. The immortalized human myogenic cells divided as rapidly as primary human myogenic cells during the early passages, and underwent myogenic, osteogenic and adipogenic differentiation under appropriate culture conditions. The immortalized cells contributed to muscle differentiation upon xenotransplantation to immunodeficient mice under conditions of regeneration following muscle injury. We also succeeded in immortalizing cryopreserved human myogenic cells derived from Leigh disease patients following primary culture. Forced expression of the three genes shortened their cell cycle to < 30 h, which is similar to the doubling time of primary cultured human myogenic cells during early passages. The immortalization protocol described here allowed human myogenic cells to recapture high proliferation activity without compromising their differentiation potential and normal diploidy.

Anchoring skeletal muscle development and disease: the role of ankyrin repeat domain containing proteins in muscle physiology

The ankyrin repeat is a protein module with high affinity for other ankyrin repeats based on strong Van der Waals forces. The resulting dimerization is unusually resistant to both mechanical forces and alkanization, making this module exceedingly useful for meeting the extraordinary demands of muscle physiology. Many aspects of muscle function are controlled by the superfamily ankyrin repeat domain containing proteins, including structural fixation of the contractile apparatus to the muscle membrane by ankyrins, the archetypical member of the family. Additionally, other ankyrin repeat domain containing proteins critically control the various differentiation steps during muscle development, with Notch and developmental stage-specific expression of the members of the Ankyrin repeat and SOCS box (ASB) containing family of proteins controlling compartment size and guiding the various steps of muscle specification. Also, adaptive responses in fully formed muscle require ankyrin repeat containing proteins, with Myotrophin/V-1 ankyrin repeat containing proteins controlling the induction of hypertrophic responses following excessive mechanical load, and muscle ankyrin repeat proteins (MARPs) acting as protective mechanisms of last resort following extreme demands on muscle tissue. Knowledge on mechanisms governing the ordered expression of the various members of superfamily of ankyrin repeat domain containing proteins may prove exceedingly useful for developing novel rational therapy for cardiac disease and muscle dystrophies.

V1a vasopressin receptor expression is modulated during myogenic differentiation

Neurohypophyseal peptides potently stimulate myogenic differentiation by acting through different receptors of the same family. Here, we show that L6C5 myogenic cells express, at a high density, a single class of V1a Arg8-vasopressin (AVP) receptor. The expression of the vasopressin receptor of type 1a (V1aR) is significantly higher in proliferating myoblasts than in differentiated myotubes. The differentiation-related decrease of V1aR expression was evident both at the mRNA and at the protein level as shown by the reduction of [(3)H]-AVP binding. However, in L6C5 cells transfected with a synthetic construct containing the luciferase gene driven by the 2 kb upstream region of V1aR, we observed a stimulation of the activity of the promoter when the cells were cultured in differentiative medium. The down-regulation of the V1aR correlated with a decreased half-life of its mRNA (half-life 5.86+/-0.74 hr in 10% fetal bovine serum [FBS] versus 3.53+/-0.72 hr in 1% FBS). Cyclosporine A and dexamethasone, but not 5'-azacytidine, treatments of cells in differentiation medium restored the V1aR level to that measured in proliferating L6C5 cells, thus confirming the role of post-transcriptional mechanisms in the modulation of V1aR expression. Taken together, these data show that mRNA stability plays a role in modulating protein expression during the myogenic differentiation process.

Biophysical Characterisation of the Small Ankyrin Repeat Protein Myotrophin

The 118 residue protein myotrophin is composed of four ankyrin repeats that stack linearly to form an elongated, predominantly alpha-helical structure. The protein folds via a two-state mechanism at equilibrium. The free energy change of unfolding in water (DeltaG(U-N)(H(2)O)) is 5.8 kcal.mol(-1). The chevron plot reveals that the folding reaction has a broad energy barrier and that it conforms to a two-state mechanism. The rate of folding in water (k(f)(H(2)O)) of 95 s(-1) is several orders of magnitude slower than the value predicted by topological calculations. Proline mutants were used to show that the minor kinetic phases observed for myotrophin arise from heterogeneity of the ground states due to cis-trans isomerisation of prolyl as well as non-prolyl peptide bonds. Myotrophin is the first example of a naturally occurring ankyrin repeat protein that conforms to an apparent two-state mechanism at equilibrium and under kinetic conditions, making it highly suitable for high resolution protein folding studies.

Binding of myotrophin/V-1 to actin-capping protein: implications for how capping protein binds to the filament barbed end

The heterodimeric actin-capping protein (CP) regulates actin assembly and cell motility by binding tightly to the barbed end of the actin filament. Here we demonstrate that myotrophin/V-1 binds directly to CP in a 1:1 molar ratio with a Kd of 10-50 nm. V-1 binding inhibited the ability of CP to cap the barbed ends of actin filaments. The actin-binding COOH-terminal region, the "tentacle," of the CP beta subunit was important for binding V-1, with lesser contributions from the alpha subunit COOH-terminal region and the body of the protein. V-1 appears to be unable to bind to CP that is on the barbed end, based on the observations that V-1 had no activity in an uncapping assay and that the V-1.CP complex had no capping activity. Two loops of V-1, which extend out from the alpha-helical backbone of this ankyrin repeat protein, were necessary for V-1 to bind CP. Parallel computational studies determined a bound conformation of the beta tentacle with V-1 that is consistent with these findings, and they offered insight into experimentally observed differences between the alpha1 and alpha2 isoforms as well as the mutant lacking the alpha tentacle. These results support and extend our "wobble" model for CP binding to the actin filament, in which the two COOH-terminal regions of CP bind independently to the actin filament, and bound CP is able to wobble when attached only via its mobile beta-subunit tentacle. This model is also supported by molecular dynamics simulations of CP reported here. The existence of the wobble state may be important for actin dynamics in cells.

Capping protein: new insights into mechanism and regulation

Temporal and spatial control of the actin cytoskeleton are crucial for a range of eukaryotic cellular processes. Capping protein (CP), a ubiquitous highly conserved heterodimer, tightly caps the barbed (fast-growing) end of the actin filament and is an important component in the assembly of various actin structures, including the dynamic branched filament network at the leading edge of motile cells. New research into the molecular mechanism of how CP interacts with the actin filament in vitro and the function of CP in vivo, including discoveries of novel interactions of CP with other proteins, has greatly enhanced our understanding of the role of CP in regulating the actin cytoskeleton.